1. Field of the Invention
The present invention relates to a fuel injection control device of an engine.
2. Description of the Related Art
In a fuel injection type engine, the basic amount of fuel injected by a fuel injector is usually calculated from the engine speed and the level of vacuum in the intake passage, or from the engine speed and the amount of air fed into the engine cylinder, and the actual amount of fuel injected by the fuel injector is feedback controlled so that the air-fuel ratio of the mixture fed into the engine cylinder becomes equal to a predetermined desired air-fuel ratio, for example, the stoichiometric air-fuel ratio, by correcting the basic amount of fuel on the basis of the output signal of the oxygen concentration detector (hereinafter referred to as an O.sub.2 sensor) arranged in the exhaust passage of the engine. Nevertheless, even if such a feedback control is carried out, when the amount of fuel injected by the fuel injector is abruptly increased as at the time of acceleration, the amount of fuel adhering to the inner wall of the intake port in the form of a liquid fuel is increased, and since this liquid fuel is not fed into the engine cylinder immediately after adhering to the inner wall of the intake port, the air-fuel mixture fed into the engine cylinder temporarily becomes lean. Conversely, when the engine is decelerated, absolute pressure in the intake port becomes low, and as a result, since the amount of vaporization of the liquid fuel adhering to the inner wall of the intake port is increased, the air-fuel mixture fed into the engine cylinder temporarily becomes rich.
Consequently, in a fuel injection type engine, the amount of fuel injected by the fuel injector is usually increased at the time of an acceleration and decreased at the time of a deceleration, so that the air-fuel ratio of the mixture fed into the engine cylinder becomes equal to a desired air-fuel ratio, for example, the stoichiometric air-fuel ratio, even if the engine is operating in a transition state such as an acceleration state and a deceleration state. Consequently, in such a fuel injector type engine, the air-fuel ratio of the mixture fed into the engine cylinder is controlled so that it becomes approximately equal to the desired air-fuel ratio, regardless of the operating state of the engine.
Nevertheless, in such a fuel injection type engine, blowby gas and lubricating oil, for example, pass through the clearance between the valve stem and the stem guide of the intake valve and flow into the intake port, and thus, when the engine is run for a long time, carbon particles, etc., contained in the blowby gas and the lubricating oil are gradually deposited on the inner wall of the intake port and the rear face of the valve head of the intake valve. These deposited carbon particles, i.e., the carbon deposit, have a physical characteristic of retaining liquid fuel, and thus, if the carbon deposit is deposited on the inner wall of the intake port, etc., the amount of liquid fuel adhering to the inner wall of the intake port, etc. is increased, and this increases the time taken by the liquid fuel to flow into the engine cylinder after the liquid fuel adheres to the inner wall of the intake port, etc. Consequently, although the air-fuel ratio of mixture fed into the engine cylinder can be controlled so that it becomes approximately equal to the stoichiometric air-fuel ratio, regardless of the engine operating state, while the engine is relatively new, if the deposit is deposited on the inner wall of the intake port, etc., after the engine has been run for a long time, since the time taken by the liquid fuel to flow into the engine cylinder is increased, as mentioned above, the air-fuel mixture fed into the engine cylinder becomes lean at the time of acceleration. In addition, since the amount of the liquid fuel adhering to the inner wall of the intake port, etc. is increased, the air-fuel mixture fed into the engine cylinder becomes rich at the time of deceleration. At this time, the air-fuel mixture becomes leaner as the amount of the deposit is increased at the time of acceleration, and the air-fuel mixture becomes richer as the amount of the deposit is increased at the time of deceleration.
Consequently, in a fuel injection control device disclosed in Japanese Patent Application No. 63-16275, when the accelerating operation of the engine is carried out, it is determined whether or not the air-fuel mixture fed into the engine cylinder is lean or rich at a predetermined crankangle, and the times of being lean and the times of being rich are calculated. Then the difference between the times of being lean and the times of being rich is calculated, and if the times of being lean are larger than the times of being rich, and the above-mentioned difference exceeds a predetermined time, it is determined that the air-fuel mixture is lean, and the amount of fuel fed into the engine cylinder is corrected and increased.
As mentioned above, if the deposit is adhered to the inner wall of the intake port, etc., the air-fuel mixture becomes lean at the time of acceleration. At this time, the time during which the air-fuel mixture becomes lean, i.e., the lean time, is almost the same, regardless of whether an abrupt accelerating operation or a gentle accelerating operation of the engine is carried out. But where it is determined whether the air-fuel mixture is lean or rich at a predetermined crankangle as mentioned above, since the engine speed abruptly becomes high when the abrupt accelerating operation of the engine is carried out, the times of the determination of whether the air-fuel mixture is lean or rich per unit of time are increased. As a result, when the abrupt accelerating operation is carried out, the times at which it is determined that the air-fuel mixture is lean are considerably increased, compared with the case wherein a gentle accelerating operation is carried out. Namely, when the abrupt accelerating operation is carried out, the difference between the times of being lean and the times of being rich is considerably increased, compared with the case wherein the gentle accelerating operation is carried out. Consequently, if the above-mentioned predetermined times for the difference are lowered to detect the lean state at the time of the gentle accelerating operation, a problem occurs in that this will cause a wrong determination that the air-fuel mixture is lean at the time of the abrupt accelerating operation, even though the actual air-fuel mixture has not become lean. Conversely, if the above-mentioned predetermined times are increased to detect the lean state at the time of the abrupt accelerating operation, a problem occurs in that this causes a wrong determination that the air-fuel mixture is not lean at the time of the gentle accelerating operation, even though the actual air-fuel mixture has become lean.